Modern multiroll calenders, in which hard, heated and soft, plastic-covered rolls are used simultaneously, could be used particularly effectively for the calendering of paper if the intermediate rolls were in each case to be lifted in their bearings to such an extent that the nip lying underneath them was relieved of the inherent roll weight. It would then be possible to set the same line pressure, from zero up to the maximum pressure, in all the nips by means of a top and bottom pressure roll, by exerting pressure on the entire roll assembly. However, this can only be realized if all the rolls in the calender exhibit substantially equal bending lines if they are held in the bearings at the journal and are bent only by their dead weight.
The proposal to equip a calender with such rolls emerges, for example, from U.S. Pat. No. 5,438,920. In that patent, the intermediate rolls are described as rolls in which the shape of the natural deflection line produced by their dead weight is substantially equal. However, it does not emerge from the patent specification how such rolls having substantially equal bending lines can be produced. This is because it is in no way trivial and is not readily possible for the average person skilled in the art, even if he masters the principal relationships between weight of a bending beam, its moment of inertia, the modulus of elasticity of the beam material and the spacing of the supports (cf., for example, Hxc3xctte, 28th Revised edition, published by Wilhelm Ernst und Sohn, Berlin 1955, pp. 876-892).
In the PCT patent application WO 95/14813, reference is also made only to the fact that the bending lines which are produced by gravity in the case of each intermediate roll have to be dimensioned such that their shapes are substantially equal. In response to the question as to how this is to be managed, the Applicant merely indicates that the intermediate rolls xe2x80x9cwere chosen in this wayxe2x80x9d. Selection methods of this type are known, for example in the case of producing balls with substantially equal diameters for precision ball bearings. However, from an economic point of view it is scarcely possible to conceive the production of a relatively large number of rolls for calender rolls and to choose from them those whose natural bending lines substantially agree.
There is a similar objective in so-called doubling calenders for producing multilayer tissue webs. In such a two-roll calender, two or more separately produced layers of fine paper fabrics are led together and pressed lightly together. This produces a muitilayer end product such as, for example, toilet paper or paper handkerchiefs. The line pressure in the nip is far lower than would be produced, for example, by simply bringing the top roll into contact. Here, too, the nip must be substantially relieved of the dead weight of the top roll. In order that the pressure profile in the nip is uniform, it is also advantageous here to use rolls with substantially coincident bending lines. The prior art here is to produce the rolls from the same material and with identical geometry, and to accept the unavoidable scatter in the material properties in terms of their effect on the bending lines. In another design, a roll which sags naturally as a result of its dead weight is combined with a further roll, whose bending line can be adapted to the first by means of an internal hydraulically acting adjustment. However, this is a complicated and correspondingly expensive solution.
In general, it is to be emphasized that rolls with equal sag in the strict sense were hitherto not available. This is based on a whole series of technical limitations:
1) Heated rolls in any form of calenders for the paper industry are almost exclusively produced with bodies made from shell-chilled cast iron. The economic production of chilled iron rolls is possible only within the framework of specific series of diameters, since each roll diameter has to be cast in a corresponding set of cast iron moldsxe2x80x94so-called chill-casting molds. These diameters are typically graduated in steps of two inches (about 50 mm). The usual diameters for multiroll calenders are accordingly, for example: 505 mm, 560 mm, 610 mm, 660 mm, 710 mm, 760 mm, 812 mm, 860 mm, 915 mm.
2) Production results in a certain tolerance range of the outer diameter of the rolls. In the industry, this is usually taken to be xc2x11% of the roll diameter. Since the maximum deflection of the roll is inversely proportional to the moment of inertia of the roll cross section, and the latter is in turn proportional to the 4th power of the roll diameter, given otherwise constructionally identical rolls, this tolerance already means a difference of xc2x14% in the sag.
3) Chilled cast iron is a so-called inhomogeneous material. The physical properties fluctuate, both on account of the composition and on account of slight differences in the structure. Material properties measured on separately cast or even concomitantly cast samples have only a restrictedly precise meaningfulness for the effective structure in the roll body itself. Deviations in the modulus of elasticity, which,can be measured precisely only to a few percent on samples, as a result of the inhomogeneity and the tolerance of the measuring method, have an inversely proportional effect on the sag. The specific density has a directly proportional influence. Material properties measured in this way cannot be used as a basis for the design.
4) In addition, a great influence on the material parameters is exerted by the cooling speed during casting, which is decisive for the thickness of the pure chilling (xe2x80x9cwhite heart ironxe2x80x9d) and the so-called transition zone. Since the pure white iron has a modulus of elasticity of about 180,000 N/mm2, and gray nodular iron typically has a modulus of about 100,000 N/mm2, deviations in the relative distribution of the two components lead to variations in the average modulus of elasticity, and thus also to a different amount of sag.
5) In the case of long rolls, there are similar variations in the axial direction, since the roll bodies are cast upright in the mold.
6) The associated polymer-covered rolls should be designed as far as possible with a diameter of the finished roll which approaches that of the heated, hard rolls or corresponds to an adjacent diameter in the standard series. It is then possible to mix hard and soft rolls in any sequence in the calender, which provides the papermaker with greater flexibility in the construction of his calender when calendering. The outer diameter of the roll core is thus defined by the thickness of the polymer layer. If the usual roll materials are used, such as gray cast iron or spheroidal cast iron, the production of such rolls with identical bending lines encounters great difficulties, since the moduli of elasticity are very different.
7) During operation, the polymer layer of the resilient rolls has to be re-machined. Depending on the layer type, it loses up to 15 mm of its thickness before being renewed. Since the layer contributes only to the weight of the roll but not to its stiffness, this means that the xe2x80x9cwornxe2x80x9d roll sags less than the new roll.
In the case of the chilled cast iron roll, this is exactly the opposite. Although the diameter of the roll is reduced only slightly when it is reground, the number of grinding operations is relatively high. During the service life of the roll, the white chilled layer is thus reduced considerably. However, since this has a strong influence on the sag of the roll, because of its high modulus of elasticity, the sag will increase gradually as this layer is ground away.
8) The modulus of elasticity both of the hard cast iron and of other iron materials depends on temperature whereas hard rolls are generally operated at temperatures around 120xc2x0 C. and higher, the polymer-covered roll bodies are in any case uniformly temperature controlled. This also results in different bending lines during operation.
9) Finallyxe2x80x94particularly in the case of polymer-covered rollsxe2x80x94differences in design are also important. For instance, rolls are bored peripherally or also implemented with a displacement body in the central bore.